1. Field of the Invention
The present invention relates to a servo device, and more specifically to a servo device for use with an optical system for reading out recorded information such as an optical video disc player and a digital audio disc player.
2. Description of Background Information
In an optical system for reading out recorded information, for example, a read out energy beam such as a laser beam is applied, via an object lens, as a light spot along a recording track formed on a recording medium. In applying the light spot of the laser beam, it is especially important to control the light spot precisely in position. Therefore, it is necessary to provide a servo device which produces a tracking error signal and also focus error signal which are to be applied to a driving means of a tracking mirror and to a driving means of the object lens. On the other hand, in order to reproduce the recorded information by means of the light spot of the read out laser beam applied on the recording medium, the read out system is provided with a photo detector operable as a photoelectric transducer, which detects a reflection of the laser beam from the recording surface of the recording medium. Further, it is necessary to use a four segment type photo detector which has, for example, four independent cells of photo diodes in a single casing, and therefore, has four independent output signals. In that case, the servo device is applied with four output signals from the photo detector to produce the tracking error signal and the focus error signal. However, in the case of the conventional arrangement of the servo device, there was a tendency that the operation of the device becomes inaccurate since the output signals from each segment of the photo detector are treated at first independently by a plurality of circuits for processing an input signal which are respectively connected to each cell of the photo detector. Furthermore, there was a drawback that the cost of the servo device tends to be rather significant because of the use of numbers of circuit elements respectively connected to each segment of the photo detector.